In an access network of a mobile network, a wireless access system that uses optical fibers as illustrated in FIG. 14 is placed in a weak signal area such as an underground mall and inside of a building where radio waves from outdoor base stations are hard to reach, as a system for providing radio waves at low cost. The wireless access system illustrated in FIG. 14 includes a center unit 101, an optical access unit 102, and a remote unit 103. The center unit 101 includes a digital baseband 104, a parallel-serial converter 105, and a serial-parallel converter 106. The optical access unit 102 includes electrical-optical converters (E/O converters) 107 and 112, optical fibers 109 and 110, and optical-electrical converters (O/E converters) 108 and 111. The remote unit 103 includes a serial-parallel converter 113, a parallel-serial converter 114, a DA converter (DAC) 115, an AD converter (ADC) 116, an up-converter 117, a down-converter 118, an antenna 119, and a crystal 120. The crystal 120 generates a reference signal required for generating a local signal for frequency conversion by the up-converter 117 and the down-converter 118.
In the wireless access system illustrated in FIG. 14, digital quadrature signals (I, Q) generated by the digital baseband 104 of the center unit 101 are parallel-serial converted by the parallel-serial converter 105, and then are transmitted to the remote unit 103 placed in a weak signal area through the optical access unit 102. Then, the signal is serial-parallel converted by the serial-parallel converter 113 in the remote unit 103, converted to an analog signal by the DA converter 115, then converted to a high-frequency signal by the up-converter 117 and emitted through the antenna 119. In reverse of the transmission, a signal received by the antenna 119 is converted to a low-frequency-band signal by the down-converter 118 in the remote unit 103, then converted to a digital signal by the AD converter 116, parallel-serial converted by the parallel-serial converter 114, and then transmitted to the center unit 101 through the optical access unit 102.
The wireless access system illustrated in FIG. 14 can be installed in various locations by placing the center unit 101 at a centralized station and placing the small and lightweight remote unit 103 in a weak signal area such as an underground mall. Considering that the global urban population is increasing, many facilities where people gather are expected to be built, and a large number of the wireless access systems will be installed in order to resolve weak signal areas in such facilities.
One example of a wireless access system in which a remote unit is further reduced in size and weight is described in PTL 1. FIG. 15 illustrates a configuration of the wireless access system in PTL 1. The wireless access system illustrated in FIG. 15 includes a center unit 201, an optical access unit 202, and a remote unit 203. The center unit 201 includes a digital baseband 204, a digital quadrature modulator 205, and a bandpass (BP) ΔΣ modulator 206. The optical access unit 202 includes an electrical-optical converter 207, an optical fiber 208, and an optical-electrical converter 209. The remote unit 203 includes a bandpass filter (BPF) 210 and an antenna 211.
In the wireless access system illustrated in FIG. 15, digital quadrature signals (I, Q) generated by the digital baseband 204 of the center unit 201 are converted to a high-frequency radio signal by the digital quadrature modulator 205, and then are converted to a 1-bit signal by the bandpass ΔΣ modulator 206 designed to minimize influence of quantization noise in a frequency band occupied by radio signals. Then, the center unit 201 transmits the 1-bit signal to the remote unit 203 through the optical access unit 202. The remote unit 203 filters the received 1-bit signal through the BPF 210 to extract the high-frequency radio signal and emits the high-frequency radio signal through the antenna 211.
When compared with the configuration illustrated in FIG. 14, the wireless access system illustrated in FIG. 15 does not require the serial-parallel converter 113, the parallel-serial converter 114, and the crystal 120 of the remote unit, thus achieving further reduction in size and weight of the remote unit.